AUXILIARY RELIEF VALVE BLOCK.- The auxiliary relief valve block (fig. 6-6) is mounted in the middle of the right trunnion support. Its purpose is to limit supercharge fluid pressure to 150 psi, limit servo fluid pressure to 450 psi, control the operation of the power-off brake, indicate by means of switches the availability of supercharge and servo fluid pressures, and provide a flow of fluid to cool the elevation and train the CAB units.

When the elevation electric motor is started, filtered supercharge fluid enters the valve block through check valve UV77 and is ported to the center of supercharge pressure relief valve UV59. From UV59, it ports to the train and elevation safety relief valves to replenish the transmission lines. As supercharge fluid builds up to its normal operating pressure of 150 psi, it forces UV59 downward against its spring and, through orifice UO4, forces supercharge pressure switch piston UC5 upward against its spring. When supercharge fluid reaches 150 psi, UC5 activates switch SIY3 indicating that supercharge fluid pressure is normal and, at the same time, UV59 moves down and opens a port that bypasses part of the supercharge fluid to the elevation CAB unit.

Figure 6-6.-AuxiIiary relief valve block (elevation motor started).

Thus UV59 maintains supercharge fluid pressure at 150 psi and bypasses the surplus fluid to circulate through, and cool, the elevation CAB unit.

Servo pressure relief valve UV58 functions in the same manner as UV59 to limit servo fluid pressure to 450 psi. Orifice UO3, piston UC6, and switch SIY4 function in the same manner as do orifice UO4, pressure switch piston UC5, and switch SIY3.

Servo fluid is ported to both train and elevation power-off brake release plungers UVT18 and UVE18, to the solenoid-operated pilot valves UVE86 and UVT86, and to the servo accumulator. Surplus servo pump output is also ported by UV58 to circulate through and cool the train CAB unit when servo fluid pressure reaches 450 psi.

Solenoids LHT1 and LHE1, located on top of the auxiliary relief valve block, set or release the power-off brakes and activate the safety relief valves. When LHE1 energizes (fig. 6-7), UVE86 shifts to the left, porting servo fluid to the bottom of UVE18. As UVE18 moves upward against spring tension, its lower land closes a port to the tank and its upper land opens a servo fluid line to power-off brake release piston UCE1. At the same time, the center land of UVE18 opens another servo fluid line to the left end of pilot valve UVE16, shilling it to the right and blocking the A-end output from the elevation safety relief valve. With the A-end output blocked by UVE16, the safety relief valve seats, allowing the A-end output to build up pressure to the B-end.

When the elevation power drive is stopped, LHE1 de-energizes and shifts UVE86 to its NEUTRAL position, porting the fluid on the bottom of UVE18 to the tank. This closes the servo fluid line and ports UCE1 to the tank, setting the power-off brake. Pilot valve UVE16 then shifts to the left because the pressure on its left end is ported to the tank and is spring loaded. This allows the safety relief valve to open and port A-end output to the tank.